panic: mutex Giant not owned (ffs_vnops.c)

Hi,

on a very -CURRENT system, I cannot use a vnode-based mdconfig(8) any
longer, the following panic (sorry but this is hand-written) occured
when I performed this command :

%%%
  # mdconfig -a -t vnode -f /boot/profile/home
  panic: mutex Giant not owned at ../../../ufs/ffs/ffs_vnops.c:350
  KDB: stack backtrace:
  kdb_backtrace()
  panic()
  _mtx_assert()
  ffs_read()
  VOP_READ_AP()
  mdstart_vnode()
  md_kthread()
  fork_exit()
  fork_trampoline()
  -- trap 0x1, eip = 0, ...
  db> show alllocks
  db>
%%%

I think I can get a kernel dump if needed (doadump() seems to work).
All I can say is that I'm pretty sure this was working around
Christmas.  IIRC, I tried various kernel dates and the earliest one
(2004.01.01) is panic()'ing as well, but I'm would not bet too much
on it (I had a few unwanted kernel source changes when I tried).

Note that I'm sending this report now because I made a `cvs up -C' so
I *know* my kernel tree is clean today.

Please, don't hesitate to ask me for deeper debugging informations.

Regards,
-- 
Jeremie Le Hen
jeremie_at_le-hen.org